This invention deals generally with condition responsive indicating systems based on disturbances of the earth's magnetic field, and more specifically with ferromagnetic object detectors for vehicle motion detection on short driveways.
Use of the earth's magnetic field as a basis for detection of ferromagnetic objects such as vehicles is not new. U.S. Pat. Nos. 3,237,155 to Brockett and 5,877,706 to Summersgill disclose such systems which are typical of many prior art devices that convert electromagnetic fluctuations into an electronic signal for producing an indication of the movement of vehicles. The typical sensor for such systems is a coil of copper wire with a large number of turns wrapped on an iron or steel core.
The input signal for such devices is generated by the distortion of the earth's magnetic field which occurs when a large ferromagnetic object such as a vehicle moves within the magnetic field. The vehicle actually bends the magnetic lines of force near it as it concentrates those lines into its ferromagnetic body. This distortion moves along with the vehicle and can best be envisioned as an electromagnetic wave motion which causes changes in the earth's magnetic field in any location as the vehicle approaches and departs from the location. A coil of wire located in such a region of changing magnetic field has a current generated in it and a corresponding voltage developed across it in accordance with the laws of electromagnetic induction. The important characteristics of the signal generated in a passive sensing coil by the motion of a vehicle through the earth's magnetic field are that the voltages are low frequency and low amplitude. However, the strength of the generated signal varies with the distance between the vehicle and the coil, the mass of the vehicle, and the speed of the vehicle. For example, as a vehicle passes the sensor coil at a typical driveway speed of 5 miles per hour, a sine wave shaped signal of approximately 1 Hz and 1 millivolt is generated. Under various circumstances, signals can be generated in coils that are from 1 to 50 feet from a moving vehicle.
These characteristics make the use of such systems quite difficult to use for driveway detection systems. To prevent vehicles on a road adjacent to a driveway from affecting a driveway system such systems are usually installed remote from the road. The generally accepted rule is that to minimize false alarms the sensor coil should be approximately 35 feet away from the road if vehicles are traveling 35 mph and 50 feet away if the vehicles are traveling over 35 mph. The result has been that such passive magnetic coil sensor based systems are rarely used on driveways less than 75 feet long.
It would be very beneficial to have such a system that would operate satisfactorily on driveways of the typical suburban housing development.